Genetic defects in the cholesterol bio-synthesis pathway lead to a spectrum of human dysmorphology syndromes with a key common theme of altered craniofacial, skeletal and central nervous system (CNS) development. A deep mechanistic understanding of how these malformations arise will require a multi- disciplinary approach. As part of this R03 to ?establish basic science-clinical collaborations to understand structural birth defects,? we will merge the expertise of three teams at the interface of genetics, embryology and sterol metabolism. We will combine clinical acumen and biochemical expertise with molecular embryology to synergistically approach the question of how sterol metabolism defects lead to brain and craniofacial birth defects. We will focus specifically on three enzymes in the post-squalene portion of the cholesterol biosynthesis pathway: hydroxysteroid (17-beta) dehydrogenase 7 (Hsd17b7), 24-dehydrocholesterol reductase (Dhcr24) and 7-dehydrocholesterol reductase (Dhcr7). These are neighboring enzymes in the latter portion of the pathway, but have very different phenotypes when ablated. The rationale for this proposal is that the molecular basis for these structural birth defects in mouse and humans has not been fully elucidated. Furthermore, detailed analysis of the mouse CNS phenotypes after birth has been prevented by perinatal death of the null mutants. Our central hypothesis is that the differing phenotypes within the spectrum of cholesterol metabolism errors occur because defects at different steps of the pathway lead to the accumulation of different sterol intermediates and/or altered lipid raft composition, which then affect embryonic development differently. We have collected and generated 5 mouse alleles (Hsd17b7rudolph, Dhcr24null, Dhcr7null, Dhcr24flox, Dhcr7flox and) to specifically address this hypothesis in this exploratory proposal. In addition, we propose to generate new tools to further develop our overarching hypothesis. We propose to address this hypothesis with two specific aims: (1). Analysis of sterols and lipid rafts in developing brains and faces upon loss of Hsd17b7, Dhcr24, and Dhcr7. (2). Determine the consequences for loss of cholesterol biosynthesis genes Hsd17b7, Dhcr24, and Dhcr7 in the cortex. The experiments in this proposal will accomplish two goals: (1) We will substantially increase our understanding of the effects of loss of three crucial cholesterol biosynthesis enzymes on CNS development. These preliminary studies and the novel Hsd17b7 allele will position us to address even more mechanistic hypotheses about the role of these enzymes, and sterol metabolism more broadly, in congenital structural brain defects, (2) We will establish an effective and demonstrable collaboration between an embryologist, a clinician researcher and an expert in sterol analysis with a shared interest in the role of sterol metabolism in structural birth defects. !